Surface tension and capillary action

In short: The surface of water behaves like a thin, stretched skin. That is why pond skaters can walk across a pond and droplets become round.

In more detail: Water molecules attract one another. Inside the liquid these forces act from all sides, but at the surface there are no neighbours above, so the molecules there are pulled inwards. The liquid therefore tries to keep its surface as small as possible.

In detail

• Round droplets: For a given volume, a sphere has the smallest surface. Small droplets and soap bubbles are therefore almost spherical.
• Pond skaters: These insects spread their low weight over water-repellent legs and do not sink, because the surface skin carries them.
• Paper clip experiment: If you lay a paper clip carefully flat on water, it floats even though metal is denser than water. A drop of washing-up liquid makes it sink at once.
• Washing-up liquid: Surfactants lower surface tension. Water can then penetrate fabrics and cracks more easily and loosen dirt.
• Capillary action: In very narrow tubes water rises by itself, because it clings to the walls and surface tension pulls the rest along. The narrower the tube, the higher it rises.
• In everyday life: Kitchen roll, sponges, towels and wicks soak up liquid through fine capillaries. Damp can also rise into masonry in this way.
• In plants: Capillary forces contribute to water transport in plants, but most of the work is done by the pull from evaporation at the leaves.
• Curved surface: In a glass, water climbs slightly at the edge and forms a curve that is open at the top, the meniscus. Mercury, by contrast, bulges upwards because it barely clings to glass.
• Connections: How surfactants work and how water behaves in its different states (seife-und-tenside-wie-sie-reinigen, aggregatzustaende).